The present invention is directed generally to liquid crystals, and, more particularly, to dopants for improving the thermal and ultraviolet (UV) stability of high birefringence liquid crystals.
High birefringence (high xcex94n) liquid crystals (LCs) are very attractive for agile laser beam steering in the infrared region and reflective displays employing cholesteric LCs. The liquid crystal material is contained between two glass substrates, called a cell. During cell fabrication, ultraviolet (UV) light is commonly used to cure the glue lines securing the two glass substrates together and to plug the hole that is left for vacuum-filling LC materials. The device may be operated at elevated temperature in order to obtain fast response time. Thus, good UV and thermal stability is an absolute requirement for a LC device to ensure long term operation.
The birefringence of a liquid crystal is determined by the molecular conjugation. As the molecular conjugation gets longer, the electronic absorption edge shifts from deep UV to blue spectral region. During UV exposure, these chemical bonds may be disintegrated. Due to the skin depth effect, the surface layers will be decomposed first. These decomposed boundary layers will disturb the bulk alignment and deteriorate the electro-optic properties of the cell.
Two known UV inhibitors, 4-methoxyphenol and 2-(2H-benzotriazol-2-yl)-4-(1,1,3,3-tetramethylbutyl)-phenol, available from Aldrich Chemical, have been added to high birefringent liquid crystals. Surprisingly, they were found to help improve the thermal stability of the liquid crystal, but did not protect the liquid crystal compound from the effects of UV exposure.
Specifically, 4-methoxyphenol, which has the structure shown below, was added to a liquid crystal mixture: 
At a concentration of 1 to 2 wt %, a noticeable improvement on thermal stability was observed. However, this additive made no contribution to UV stability at all.
Another longer conjugated chemical, 2-(2H-benzotriazol-2-yl)-4-(1,1,3,3-tetra-methylbutyl)phenol, was also tried. Again, no improvement on UV stability was detected. This compound is so bulky that adding 2 wt % to PTTP-24/36 caused the viscosity to increase about 10%, which reduced the response of the liquid crystal cell. (The PTTP-24/36 liquid crystal mixture is described in greater detail below).
Thus, there is a need for preventing disruption of chemical bonds in liquid crystal molecules as a result of UV exposure.
In accordance with the present invention, specific dopants are provided that protect high birefringence liquid crystals from UV and thermal degradation. These dopants are called xe2x80x9camino-tolanexe2x80x9d compounds and have a structure given by: 
where Rm and Rn are each independently an alkyl group or an alkoxy group having from 1 to 12 carbon atoms or an alkenyl group or an alkenyloxy group having from 2 to 12 carbon atoms, X is selected from the group consisting of NO2, CN, NCS, F, CF3, and OCF3, and Y1 and Y2 are each independently H or F. The concentration range of the dopant in the liquid crystal is about 1 to 20 wt %.
The advantages of the use of the nitro-amino-tolanes are numerous, including their low cost, their birefringince is as large as 0.4, their dielectric anisotropy (xcex94∈) exceeds 50, and the viscosity of the mixture of the dopants with the liquid crystal is low.